Enough With the Solar Power 'Magic,' Please!

Free power from the sun is a great idea, except when it's not. Often such schemes make little or no sense, and are mostly attention-getting ploys that we love to read about, apparently.

I'm an advocate of solar power -- when it makes sense. For energy harvesting in data loggers, site-based power, spacecraft, and large fixed installations such as houses, for example, it often works out fairly well, or is the only viable solution.

At the same time, I get annoyed when the promise of free solar power is used as an attention-getting gimmick, regardless of how impractical or nearly useless that promise is. The latest example of this is a piece I just saw in The Wall Street Journal, "Ford to Show Solar-Powered Hybrid at CES." (Sorry, it may be behind the paywall.) In brief, Ford is equipping some of their hybrid C-Max concept-car models (based on a commercially available vehicle) with solar panels on the roof, along with some sort of sun-tracking system. It's not clear. It looks like it may have steerable Fresnel lenses in order to provide both basic and booster free power.

What I like to do when confronted with this sort of technology claim is perform a quick back-of-the-envelope, order-of-magnitude sanity check. I have a notepad I made just for this function, to use at concept meetings. It's made entirely of envelopes. (See photo.) My analysis: I don't care how good that solar power system is, it won't collect enough power to make it even remotely worthwhile.

A few rough numbers tell the solar tale. On the source side, the maximum solar radiation reaching Earth's upper atmosphere is about 1,000 W/m2, according to various reputable sources. That's the maximum. Just factor in atmospheric losses, clouds, the angle of the sun at different latitudes, and seasonal effects, and that number drops down rapidly.

On the conversion side, you have the efficiency of the solar cells reaching say, 15% at best, minus losses in the dc/dc power system of around 50%. (Remember, this is all an estimate. We are just trying to see where things stand.)

A car roof is perhaps four square meters -- let's say five square meters, to be generous. If the car roof captures full-intensity solar radiation, you've received 5,000 watts per surface, less the capture/conversion losses. Work those losses in and you may garner 375 W for the car battery to store, assuming you got the full 1,000 W/m2 at the cell surface, which won’t happen.

To make a long story short, you'll be lucky to collect a few hundred watts. Now look at the load: One horsepower is about 750 W, and you've collected less than half that amount. How far can you go, then? Although it depends on how long you've been able to collect that energy, the real answer is "not far at all." Well, perhaps you can use it to supplement the car's internal power source? Again, not much help. A car today can use it up easily for all the infotainment subsystems. The bottom line is you are putting a lot of cost and stuff into the car for very little useful return.

The power-harvesting reality is simple, determined by the laws of physics. You collect energy over time, when it is available, but you spend it quickly, as power, to meet the much higher needs of the load. Just because you are collecting it at a slow rate doesn't mean you can get away with spending it at that same rate, since most real-world loads have specific minimum-power requirements to make them functional. (See Energy Availability vs. Power Needs if you need a refresher.)

Ford's announcement worked. It got them attention. I'll give them credit for that. But the solar-powered car story is an oldie but goodie. It always amazes me that it still works. (Check Popular Science from the 1950s and you'll see the same story!)

Have you seen any free-power-harvesting schemes that made little or no sense? Have you ever tried to point this out to proponents?

@Henry, rtapl. I went back and watched the Ford video again. They claim 1.5 m2 of panels and 300W output - that's 200w / m2 or 20% efficiency.

The "Carport" is shown as 4.3 x 5.3 m = 22.8 m2 of concentrator lens, that's 15.2 times the area of the panels, and they are claiming an 8x increase in power as result. Considering the difficulty in precisely positioning the car, that's probably reasonable, but I'd still question whether you can drive any solar panel that hard? And that's a big carport for a small car, I wish my garage at home was that big...

Again taking their figures, not ones I've estimated, that gives 300W x 8 x 5 hours - 12 KWH. Depending on the driving, that would give up to a bit more than an hour of driving per day. So it would be feasible for shortish commutes.

Today converter don't have a 50% conversion ratio but more around 80% to 90%

It's not the 1.5m² of the solar cell surface you have to take into account, it's the canopy surface (around 20m²) that will be concentrated onto the car roof top.

The solar cells used there are multilayer type that can convert that amount of concentrated solar energy to electricity. Ford says 8x concentration, so not all of the 20m² equivalent surface will be converted, but more around 10m² with 20% ratio.

Those cells are already working as prototype with real 8x factor, but there's still the self-heating issue to take care of.

Nothing magic here.

So you can full-charge a car battery in one day... if the battery is not too big. If you want a bigger car and/or longer range then it will work for 50% of the full charge, that's is still good to have.

The stupid thing in this business model is that you'd better have 50m² of standard solar cells on your house roof top and charge your car with them, it will be cheaper.

@rtapl123 The last (relatively expensive) panels I bought were a little less than 15% efficiency. Sunpower panels ar about 16% while Sanyo panels are around 17.25% efficient. The cheap panels are a lot lower efficiency. Concentrators are an interesting option, but it's pretty easy over power a panel - especially during winter cold.

Somehow the author has his head in the sand. 15% is not the max efficiency for even cheap cells these days. Concentrators are typically over 30%. Does anybody check these articles before publishing them?

I've also lived off-the-grid for the past 9 years and I totally agree with the points in this article. However, I will throw out one small point in favor of the concept. With more and more electronics installed in the car, or potentially plugged into the car these days, it's nice to have a modest power source that doesn't require the car to be running. Dead batteries would be a thing of the past! I've often wished for an integrated solar charger for my car, but granted, this usage doesn't require or even justify a lot of fancy technology.

I drive a LEAF, and it is solar powered. But indirectly of course. I have 6kW of solar panels on my home roof. The electricity produced is put back on the grid. Thanks to net metering I get credit for that and help produce power during higher demand times of the day. Then I charge my car at night when demand is low. Eliminates thestorage problem for now. I hoping the organic/inorganic flow cell batteries solve that significant problem.

My LEAF gets 4 miles per kWh on average with about half my driving on the highway.

In December, the solar array produced 25 to 70 kWh per week (The snow is NOT helping, the array is at a low roof angle so it doesn't slide off, takes a while to melt.)

I use about 30kWh per week driving, so plenty of excess to help power my home. I'm hoping that in the other three seasons most of the house power is covered too.

Thanks to rebates (50% from utility, 30% tax) I'll pay about $8,000 or less for the system. It should payoff in 5-10 years depending on how fast electric rates rise, and meanwhile I'm not driving my car on coal.

@majortom84 - if you are looking at start-stop city driving I think the consumption would go up if anything - though you do have zero consumption at traffic lights and hopefully regenerative braking helping a bit. In suburban or industrial areas you could have parking lots covered in carports with fresnel lenses but in inner cities it would be more difficult.

No holes to poke, but I do wonder if the numbers look any better if you assume 25-30mph, say for city driving. (Heck, if I were stuck in the desert I would be happy with 10mph.) Wind resistance is much less than half, I think. Yes, of course in a city it is more difficult to come by direct sunlight.